Gaga said the injury forcing her to postpone shows in Detroit and Chicago is synovitis, a severe inflammation of the joints.

"Synovitis is an inflammation of the membrane that lines a joint capsule," said Dr. Jennifer Ashton, senior medical contributor for ABC News. "It can cause significant pain, significant swelling and reduced range of motion."

"The first course of action is rest and then some medications to reduce the inflammation," Ashton said.

The "Just Dance" singer gave no details about how she suffered the injury but said it was her show in Montreal, one of the 90 concerts in over 40 countries she's performed, on Monday that forced her to come forward with her injury.

"…After last night's performance I could not walk and still can't," she tweeted. "To the fans in Chicago Detroit & Hamilton I hope you can forgive me, as it is nearly impossible for me to forgive myself. Im devasted & sad."

Lady Gaga's representatives confirmed the tour will be postponed through Sunday because of her ailment but did not specify her treatment or prognosis for recovery.

"Lady Gaga will not go on unless she will give a jaw-dropping performance," Raha Lewis, a writer for People magazine, told ABC News. "To have to postpone four shows, you know that she's not doing well."